1. Field of the Invention
This invention relates to a stringiness-preventing pad attached to a mold in an injection molding machine for preventing a molded object from stringing when removing the molded object from the mold.
2. Description of the Prior Art
When a molded object is removed from a mold upon solidification of molten resin material injected into the mold through an injection cylinder nozzle of an injection molding machine, a sprue slug solidified in a sprue of the mold remains connected with the molten resin material in the cylinder nozzle through stringy molten resin. This phenomenon is the so-called "stringiness" inevitably caused by the viscosity of the molten resin material. To be specific, as shown in FIG. 1, the sprue slug s which is formed in a sprue 3 and connected with a desired object molded in the mold cavity of the mold 1 has incidentally a stringing part d dangling from the sprue slug. The stringing part d to be ultimately discarded is formed by the molten resin material in the injection cylinder nozzle which is stickily attached to the sprue slug s solidified in the sprue 3 when the sprue slug is removed from the mold 1 upon completion of one molding shot. The stringing part d entails problems such as difficulty in removing the united molded object and-sprue slug s from the mold 1 and misalignment at the parting face 7 formed between stationary and movable dies constituting the mold in mold-closing. In some cases the stringing part may possibly mar up the mold 1, thereby cause dimensional distortion or surface blemish on the molded object obtained finally obtained.
A conventional means has been proposed for cutting the stringing part attached to the sprue slug in Japanese U.M. Public Disclosure SHO 62(1987)-119212(A), which is provided with a slidable cutting plate capable of cutting out the stringing part. However, this conventional device disadvantageously is complicated in mechanism, impedes the continuity of injection molding and involves decrease of injection speed. Besides, the cutting plate of the conventional device frequently fails to remove the stringing part from the molded object.
Another prior art mold structure is disclosed in Japanese U.M. Publication SHO 63(1988)-50102(B), in which a metal thin plate being high in heat conductivity and having a fine hole communicating with the sprue is attached to a sprue bush so as to be held between the nozzle touching face of the sprue bush and the injection cylinder nozzle. The metal plate serves to quickly radiate the heat of the molten material injected into the sprue so that the molten material in the sprue is rapidly cooled and solidified to prevent the molten material in the sprue from thermally stretching. To remove the stringing part in this conventional mold structure, the molded object including the sprue slug is separated from the molten material in the cylinder nozzle at a contracted part formed by the fine hole bored in the metal plate by forcibly disconnecting the injection cylinder nozzle from the sprue bush. This conventional structure has a disadvantage in that the molten resin material would be reluctant to pass through the fine hole in the metal plate, resulting in short molding and decrease of injection speed.
Otherwise, a method for preventing stringiness by controlling injection pressure or providing a check valve at the nozzle tip of the injection cylinder nozzle can be envisaged. Nevertheless, a possible mechanism for practicing the aforenoted method would be complicated in structure awkward to handle and easily brings about a malfunction.